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Math Help - Simple Intrest

  1. #1
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    Simple Intrest

    A sum of money doubles itself in 7 years.In how many years will it become 4 times at SI
    (a) 15
    (b)21
    (c)18
    (d)25
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  2. #2
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    Call the sum of money x.

    So if it doubles in 7 years then

    After 7 years it becomes 2x

    2x becomes 4x after another 7 years which is four times the original ammount.

    So i would have said that the answer would be 14 years but maybe im missing something.
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  3. #3
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    Quote Originally Posted by Niall101 View Post
    Call the sum of money x.

    So if it doubles in 7 years then

    After 7 years it becomes 2x

    2x becomes 4x after another 7 years which is four times the original ammount.

    So i would have said that the answer would be 14 years but maybe im missing something.
    I also tried it the same way.But answer was given as 21.
    Any way thank you.
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  4. #4
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    ah yes i see the problem I used compound interest.

    simple interest you do no get interest on the interest.

    So if sum of money = x

    Every 7 years you will get x interest

    after 7 years you have x + x

    After 14 years you have x + x + x

    After 21 years you have x + x +x +x =4x
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